The hypothesis that extrastriate visual areas in mature monkeys have the capacity to extensively reorganize retinotopically and functionally after partial lesions of V1 will be evaluated. Chronically implanted microelectrode arrays will be used in MT to determine the time course and occurrence (if any) of recovery of the response properties of single neurons after partial lesions of V1. Possible outcomes include: 1) abolished or reduced responsiveness that does not recover; 2) recovery with reactivation depending on inputs from the remaining portion of V1; 3) reactivation depending on LGN or superior colliculus relays to extrastriate cortex; or 4) some combination of these alternatives. The possibilities of reorganization or enhanced use of preserved pathways have major implications for clinical studies and patient therapies. Cortical reorganization may account for behavioral recoveries after limited lesions of MT (Newsome and Pare, 1988), Yamasaki and Wurtz, 1991), blindsight after extensive lesions of V1 (Weiskrantz, 1996), and perceptual errors (e.g. Ramachandran and Gregory, 1991). The chronic array study will be supplemented with standard microelectrode maps of MT several months after lesions of half of V1 or half of MT. These experiments will allow us to more accurately determine how much MT retinotopically reorganizes, if at all, after such lesions.